Transmission lines are important elements in microwave circuit applications. These devices provide the interconnection between active and passive devices of microwave circuits, and are also utilized as impedance matching elements. A microstrip line is a type of transmission line widely utilized in monolithic microwave integrated circuit (MMIC) applications.
Microstrip lines have a number of advantages when utilized in MMIC applications. First of all, since microstrip lines are formed of conductive planes disposed on substrates, these devices are readily adaptable to the manufacturing process of the integrated circuits. Accordingly, microstrip lines may be integrated on the same substrate as commonly used integrated circuits such as complementary metal-oxide-semiconductor (CMOS) circuits.
Generally, microstrip lines comprise a signal line over a ground plane, which is a solid metal plane, with a dielectric layer or layers separating the signal line from the ground plane. The ground plane has the advantageous feature of isolating the signal line from the substrate, hence any substrate-induced losses are reduced. However, the formation of the ground plane also incurs drawbacks. As the scaling of backend processes continues to trend downward, the vertical distance between the signal line and the ground plane becomes significantly smaller; this requires the signal line to be increasingly narrower in order to achieve the desired characteristic impedance. Consequently, ohmic losses in microstrip lines become increasingly more significant, and demand better impedance matching between microstrip lines and network devices. Furthermore, the ground plane becomes a barrier for tuning the characteristic impedance of microstrip lines; this is due to the limited vertical distance between the signal line and the ground plane, a small distance with little room for tuning.
In addition, microstrip lines typically occupy a large chip area. For example, the electro-magnetic wavelength in SiO2 dielectric material is about 3000 μm at 50 GHz. Accordingly, microstrip lines have a requirement that the length of the microstrip line be at least a quarter of the wavelength, here about 750 μm, in order to match network impedance. This is considered to be area-consuming. With the increasing down-scaling of integrated circuits, the chip-area requirement of the microstrip lines becomes a bottleneck that prevents the integration of microwave devices and integrated circuits adopting CMOS devices.
Unfortunately, the microstrip line structure also has limited application in radio frequency (RF) circuits, particularly in microwave and millimeter wave integrated RF circuits like those in GPS satellite systems, PDA cell phones and ultra-wideband (UWB) wireless communication systems. In order to complement the gain-bandwidth of silicon transistors, microwave applications require passive devices with a low parasitic loss that can be isolated from other circuit sub-blocks. The parasitic loss of RF on-chip components cannot be scaled as readily as the parasitic loss that accompanies active devices such as transistors.
Additionally, to implement many RF circuit designs it is necessary to use four port components, a four terminal device wherein an input into any one terminal will appear on the other three. A single signal line microstrip transmission line can only accommodate two-port components, a two terminal device with an input terminal and an output terminal. Accordingly, what is needed in the art are mechanisms that overcome the deficiencies of the prior art.